In the related art, in a device including a rotation body, a metal bearing is mounted in a rotation shaft in order to reduce frictional resistance which occurs by rotation of the rotation body. For example, a bearing is also mounted in a rotation shaft of a rotation body in a motor for rotating the rotation body (so-called rotor) by using an electromagnetic force or in a power generator for rotating a rotation body (so-called turbine) by water power and the like, so as to generate power.
In the motor, the power generator, or the like, there is a problem in that a current leaks into the bearing and corrosion (referred to as electrolytic corrosion) occurring by flowing of the current easily proceeds. Thus, a technology for preventing an occurrence of electrolytic corrosion in a bearing is examined.
For example, Patent Document 1 discloses a technology in which any of an outer ring and an inner ring which are made of metal and are disposed at concentricity of a bearing is coated with a sprayed ceramic layer, so as to perform electrical insulation, and thus a flow of a current is blocked and electrolytic corrosion is prevented. In this technology, thermal spraying is performed by using a powder mixture in which powder of titania (TiO2), silica (SiO2), and chromium oxide (CrO2) is mixed to powder of alumina (Al2O3), and sealing treatment is further performed so as to form an insulating layer. That is, the technology is as follows. A mixture having a low melting point is mixed, and thus a gap in an insulating layer of alumina (Al2O3) which is the main raw material is filled. Thus, porosity is reduced and variation thereof is suppressed, and thus stable insulating properties (that is, electrolytic corrosion resistance) for a bearing are obtained.
However, in the technology disclosed in Patent Document 1, the thickness of the insulating layer is required to be equal to or more than 250 μm for ensuring necessary insulating performance and coating strength. As a result, there is a problem in that thermal conductivity of the bearing is degraded. If the thermal conductivity is degraded, heat generated by frictional resistance occurring by rotation is accumulated in the bearing, and this is the cause of reducing the lifespan of the bearing. In a case where an insulating layer is formed by thermal spraying so as to be thick, there is a problem in that an operation period becomes longer and manufacturing cost of a bearing is increased.
That is, a technology of manufacturing a bearing which includes an insulating layer having high coating strength, and good electrolytic corrosion resistance and thermal conductivity at low price is not established yet.